Tag Archives: TESS

A major limitation to the discovery rate of extrasolar planets is the hardware available with which to detect them. Instruments like HST, Spitzer and Keck have revolutionized astronomy, and have provided a major source of extrasolar planet science — both in atmospheric characterisation and planet detection. These instruments are not dedicated to extrasolar planets, though, and time on the instruments must be shared with astronomers investigating cosmology, interstellar dust, galactic structure, and dark matter, to name a few. There is considerable interest in dedicated exoplanet science instruments – instruments that were designed to do extrasolar planet science, as opposed to instruments and spacecraft designed years ago that we have been fortunate enough to be able to torture exoplanet data out of.

One such dedicated observatory is the Automated Planet Finder, a robotic 2.4 metre telescope whose task is to search for extrasolar planets around nearby stars with a Doppler precision of ~1 m s-1. It observes ten starts a night, and will observe about a thousand stars in the solar neighbourhood. By now, the APF has been in service for a few months, but its data has already been crucial in confirming a new four-planet system of gas giants at HD 141399, and a Neptune-mass planet at GJ 687 (.pdf link) which is only 4.5 pc away.

Automated Planet Finder

The age of the dominance of Doppler spectroscopy in the discovery of new planets is clearly over. Doppler spectroscopy has fallen to second place behind transit detection, largely as a result of Kepler‘s superb performance and its discovery of over 3,000 planet candidates, as well as a change in strategy by ground-based Doppler surveys. The discovery of planets are no longer interesting for the most part, so important assets are being targeted away from large surveys of bright, metal-rich stars in the hopes of detecting intermediate- to long-period Jovian planets, and toward focoused observations of nearby stars in the hopes of detecting low-mass planets that more closely resemble our solar system. The interest now is in attaining higher precision spectrometers to observe nearby stars to try to detect very low mass planets. This takes a lot of time, and explains both why the nature of discoveries from Doppler spectroscopy has changed in recent years, as well as why the discovery cadence has trailed off. As we move away slightly from discovery for the sake of understanding the underlying planet population distribution, and toward looking for nearby targets for future follow-up, the motto is quality over quantity.

However, the outer regions of planetary systems have been largely unprobed, as Doppler surveys and transit surveys are both biased toward short-period planets. Microlensing results have shed some light on the planet population distribution at higher separation, comparable to the outer Solar System, and it seems that the abundance of gas giant planets picks up quite a bit, but this will need to be confirmed. Fortunately, new instruments are coming online that will help address these issues.

The Gemini Planet Imager on the Gemini Observatory is another new toy that has come to light. It will be used to conduct a 890 hour survey of ~600 stars from 2014 to 2016, and this January they posted first-light images, including one of β Pictoris b.

Beta Pictoris b

Another imaging instrument that is about to contribute to extrasolar planet science is SPHERE for ESO’s Very Large Telescope, capable of detecting giant planets with orbital radii >5 AU. It will observe nearby young star associations with ages of 10 – 100 Myr within 30 – 100 parsecs, as the planets of these stars will be both bright in infrared due to their age, and well separated from their star due to their proximity. All stars within 20 parsecs will also be observed, as well as stars with known long-period planets. First light is expected to be soon in 2014, perhaps as early as within the month.

A more long-term instrument of interest is ESO’s ESPRESSO spectrograph, also on the VLT. ESPRESSO will surpass the highly successful HARPS spectrograph. With a required accuracy of 10 cm s-1 (and a goal of a few cm s-1), It is expected to be sensitive to terrestrial and Earth-like planets around sun-like stars as faint as V ~ 9. First light is expected to occur in 2016.

With new planet transit search missions such as TESS and PLATO to identify transiting planets around bright stars, and improved spectrographs for mass and density determination of those planets, in addition to new direct imaging instruments and astrometry with the GAIA mission to probe the population statistics of the middle to outer regions of planetary systems, the post-Kepler era is promising to be quite an exciting, with a diverse array of complementary instruments working together to further illuminate the nature of planetary systems in the Galaxy.

The Kepler spacecraft is a wonderful asset. Despite its ailing health, it has managed to provide us with 3,000 candidate planets which permit a statistical look at the census of planets in the local Galaxy. We’ve learned planets are actually pretty common – they’re the rule rather than the exception. But for all of its contributions to science, and they have been numerous, the candidate planets Kepler has found tend to orbit stars that are far away. This is because the spacecraft stares at the same patch of sky continuously. If you take any patch of the sky, there will be far more distant stars than near stars in that patch because the volume of space within that patch increases with distance. So while the Kepler results are splendid for estimating the frequency of planets, they are not particularly useful for providing us with targets for follow-up study.

What we need is a mission to identify transiting planets around nearby and/or bright stars. It seems NASA has answered the call for such a mission and has, as of today (April 5, 2013), selected the TESS mission for development and launch in 2017.

The Transiting Exoplanet Survey Satellite (TESS) was conceived to address this problem. It will observe an area of sky 400 times that of Kepler at a time, studying two million stars with brightnesses V < 12, as well as the closest 1,000 M dwarf stars — essentially all red dwarfs within 30 pc. The mission will last two years and will uncover perhaps 2,000 planet candidates, a few hundred of which could be Earth-sized. The number of discoveries could be as much as from Kepler, but around nearby and bright stars where these planets would actually be feasible to study and examine for the possibility of life with transmission spectroscopy. Ground-based spectrographs would have a much easier time confirming the planets with Doppler spectroscopy.

It’s important to note that TESS will not stare at the same part of the sky for an extended time to detect Earth-analogues as Kepler does. TESS will move from one part of sky to another, observing each part for only a few months, so the orbital periods of discovered planets will be on the order of weeks – perfect for habitable planets around M dwarfs, but less so for habitable planets around G or K dwarfs. TESS’s wide, shallow approach to finding planets nicely complements Kepler‘s narrow, deep search for planets.

The study of transiting planets around M dwarfs is a worthwhile pursuit. M Dwarfs emit most of their energy in the infrared, where absorption lines of water and carbon dioxide reside and are prominent. The planets will necessarily be in shorter period orbits. This is hugely exciting for extrasolar planet science. It’s quite possible that the first extrasolar biosphere, and perhaps even the first extrasolar planet that is visited by a human spacecraft in the distant future, will be discovered by TESS. It is my opinion that this spacecraft is much more exciting than even Kepler.